change Rgraphum::Graph#load easy to add load strategy.

To add new strategy, add following method:

    Rgraphum::Graph#build_graph_from_#{format}(graph, options)

ex)

    Rgraphum::Graph.load(format: :my_builder, options)

This invoke following method.

    Rgraphum::Graph#build_graph_from_my_builder(graph, options={})